Magnetic separation utilizes the force of a magnetic field co-acting with some other force to produce differential movements of material through the field. Fundamentally, differences in magnetic permeability of material constitute the basis for separation, but such separation is influenced by mechanical attributes of the separator used. Generally, magnetic permeability refers to the measure of the ease with which magnetic properties may be induced by the action of a magnetic field. Mixtures susceptible to magnetic separation are generally those which include materials that fall into different classifications, e.g., strongly magnetic and weakly magnetic materials versus non-magnetic materials. As used herein, the term "magnetic" refers to material which is magnetically susceptible, and is not meant to necessarily imply material which may be permanently magnetized.
Magnetic separation is typically used in association with iron mining operations. For example, in instances where the iron ore is of relatively low grade and contains a large quantity of extraneous rock material, or gangue, magnetic separation may be used to separate the iron ores from the gangue or other heavy minerals of a slurry. An example of such an operation is the separation of magnetite from taconite. Such separation may even be performed on tailings from mining operations which may be a valuable source of iron.
Separation of solids according to their magnetic properties is known, and various apparatus are described for performing such a separation function. For example, the Conklin wet magnetic separator is described in the book entitled, Ore Dressing, by R. H. Richards, Vol. II, 2nd Ed., Section 591, p. 810, McGraw-Hill Book Co. (1908). The Conklin wet magnetic separator is described as including a distributing belt placed on an incline. Magnets for providing a magnetic field are placed beneath the belt. In other words, the belt is on an incline and the magnets are placed beneath the belt between an ascending and descending side of the belt. Ore is fed upon the belt near the magnets generating the magnetic field. A stream of water runs down the belt and carries the non-magnetic material off at the lower end of the belt, while the magnetic particles are held against the belt by attraction resulting from the magnetic field. The magnetic particles are carried by the belt as it moves up the incline and are discharged as the belt starts to decline at an upper end thereof.
In addition, a Roche wet belt machine is described in the book entitled, Handbook of Mineral Dressing, by A. F. Taggart, Sect. 13-20, John Wiley & Sons, Inc., New York (1945). The Roche wet belt machine operates in a manner similar to the Conklin wet magnetic separator. The Roche wet belt machine consists of an endless rubber vanner belt set on a slope of 10.degree. to 80.degree.. A battery of electromagnets having alternating poles of opposite polarity are enclosed in a box and positioned underneath the upper run of the belt which is held at the incline. Feed, e.g., magnetic material and non-magnetic material, is introduced at a lower portion of the inclined belt and magnetic material is held against the belt by the action of the magnets positioned thereunder. The magnetic material is carried up the slope of the belt against a stream of wash water supplied at an upper end of the belt. The nonmagnetic material, acted on by the wash water and gravity, flows down the slope and discharges over the lower end of the belt. Washing of the concentrate is aided by the winnowing motion caused by the alternating polarity of the magnet poles of the magnets positioned underneath the belt.
Both the Conklin and Roche wet magnetic separators generally collect a thin layer of magnetic material on the generally flat inclined belt thereof. Such a thin layer of magnetic material being captured is generally inefficient with regard to throughput for the feed or mixture to be separated. In both the Conklin and Roche wet magnetic separators, the collected magnetic material tends to collect over the upper-most magnet positioned underneath the inclined belt. The magnetic material tends to remain stationary at this position because the magnetic material tends to slide backwards on the belt as the belt travels up the incline beneath it. As such, erratic discharge of the collected magnetic material over the upper edge of the inclined belt results. In other words, such problems lead to a non-uniform discharge as the inclined belt passes over the upper edge of the magnetic separator. This non-uniform discharge in turn interferes with efficient operation of the magnetic separator.
Use of a ferromagnetic matrix to assist magnetic separation has also been described. For example, in the publication, Handbook of Mineral Dressing, by Taggart, Sect. 13-21, the Frantz Ferro-filter is described. The Frantz Ferro-filter uses a set of magnetic screens to assist in separation. Further, also described in the Handbook of Mineral Dressing by Taggart, Sect. 13-21, is a magnetic trough separator which uses rectangular auxiliary pole pieces to assist in magnetic separation. The magnetic trough separator includes a battery of magnets housed in a waterproof copper casing over which a removable tray is placed. The removable tray has a bottom studded with the rectangular auxiliary pole pieces; the pole pieces appropriately staggered. The tray is shown at an incline and pulp is fed from a pipe into the removable tray such that the pulp flows downhill and discharges into a suitable launder. During its downhill run, the pulp is subjected to the magnetic field induced in the auxiliary pole pieces with magnetic contaminants being attracted and held to the auxiliary pole pieces. The magnetic field is then cycled off and the removable tray is removed from the separator to remove the magnetic contaminants therefrom and the tray is then repositioned in the separator. For example, the separator may be used for removal of small amounts of magnetic material from clay, silt, and the like.
The Frantz Ferro-filter and the magnetic trough separator both have problems associated therewith. For example, neither of such apparatus provide means of continuously discharging collected magnetic material. To remove the magnetic material from such apparatus, the flow of the feed slurry or mixture must be interrupted. For example, the electromagnets used to magnetize the magnetic screens in the Frantz Ferro-filter must be cycled off to remove the collected material and the tray of the trough separator must be removed to obtain the collected magnetic material. Such cyclic operation is generally impractical. Particularly, such cyclic operation becomes impractical when the feed or mixture being separated contains more than a very small percentage of magnetic material. In other words, if a large percentage of magnetic material is being captured, the separators must be interrupted constantly to remove such collected magnetic material.
Continuous collection and discharge of magnetic material from a matrix in a magnetic separator is described with reference to the Jones separator in the SME Mineral Processing Handbook, by N. L. Weiss, Vol. 1, pp. 6-36 to 6-38, SME (1985). The Jones separator, and other similar machines, is made continuous by using the basic design of a Forsgren separator in which the slurry enters a high magnetic field region between magnet pole pieces in a rotating annular ring containing a high gradient collecting matrix for attracting magnetic material of the slurry provided thereto. The annular ring is generally held in a horizontal position. The non-magnetic material passes quickly through the matrix and the magnetic material is removed when the ring moves out of the high magnetic field region between the magnet pole pieces. Such an apparatus may be referred to as rotating annular ring or "Carousel" design.
In such Carousel designs, slurry and wash water flow is approximately perpendicular to the travel direction of the annular ring which rotates in a horizontal plane. The rotation of the collection matrix in the annular ring physically diverts the flow of slurry in the direction of matrix travel. As the rotation rate of the annular ring increases, an undesirable increasing percentage of the non-magnetic material in the feed or slurry is diverted into the launders which are supposed to collect only magnetic material flushed off the collection matrix by wash water. Further, another undesirable effect encountered with the Carousel designs is permanent entrapment of particles within the ferromagnetic matrix. This is particularly problematic with regard to strongly ferromagnetic and oversized particles. In other words, since the matrix remains fixed in a position within the Carousel design, and further since the dislodgement force that can be applied by spray water is attenuated within the body of the matrix, once particles become strongly entrapped in the matrix, it is very difficult to remove them. As trapped magnetic particles build up in the matrix, the capacity and performance of the machine deteriorates. Eventually, the only solution is the very costly step of shutting down the machine, disassembling the matrix, and cleaning it.
Generally, Carousel-type machines use electromagnets to generate the desired magnetic fields. A modified Carousel apparatus using permanent magnets is described in U.S. Pat. No. 3,947,349 to Fritz, entitled "Permanent Magnet High Intensity Separator," issued Mar. 30, 1976; U.S. Pat. No. 4,046,680 to Fritz, entitled "Permanent Magnet High Intensity Separator," issued Sep. 6, 1977; and U.S. Pat. No. 4,874,508 to Fritz, entitled "Magnetic Separator," issued Oct. 17, 1989. The original described Fritz separator apparatus incorporates various changes from the conventional Carousel design, several of which tend to reduce the problem of matrix entrapment or clogging as described above. In the Fritz apparatus, the axis of the rotating cylinder is set horizontally, causing the annular rings and matrix thereof to rotate in a vertical plane as opposed to rotating in a horizontal plane as described above. The direction the slurry flow is then vertical in the plane of the ring. Further, instead of a single ring, Fritz describes the use of a series of side-by-side rings separated by regions containing permanent magnets and vibration is applied to assist in dislodgement of trapped particles.
As in conventional Carousel-type designs, the slurry flow in the Fritz apparatus is in the downward direction and approximately at a right angles to the matrix travel of the apparatus. The magnetic material is flushed off the matrix after the ring has rotated approximately 180.degree. from the feed or slurry inlet. Because flush water flow is also downwards, the matrix is subjected to a back flush action which is beneficial for removing matrix entrapped particles that were trapped simply because they were too large to pass through openings in the ferromagnetic matrix. However, the Fritz machines, despite the vibration and the back flush action, do not adequately reduce the matrix clogging problem.
In U.S. Pat. No. 4,874,508 to Fritz, such matrix clogging is further addressed by providing flexible race walls which are deformed to expand and recompress the matrix to facilitate separation of magnetic material from nonmagnetic material. For example, deformation of the race walls is accomplished by a cam mechanism. The complex design of the Fritz machine leads to generally high construction costs. Particularly, such costs increase as the magnetic separator is scaled up to larger commercial sizes. For example, the construction cost per foot of width for the magnetic separator of Fritz remains essentially the same as the width is increased. In other words, a 10 foot wide unit will cost nearly ten times as much as a 1 foot wide unit. This is because most of the elements in the 1 foot unit must be duplicated ten times to make a 10 foot unit. As such, complexity in design is a particularly undesirable aspect of magnetic separators.